Human Immunodeficiency Virus (HIV) (cont.)

Mary D. Nettleman, MD, MS, MACP

Mary D. Nettleman, MD, MS, MACP is the Chair of the Department of Medicine at Michigan State University. She is a graduate of Vanderbilt Medical School, and completed her residency in Internal Medicine and a fellowship in Infectious Diseases at Indiana University.

Charles Patrick Davis, MD, PhD

Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

How do health-care professionals diagnose an HIV infection? What are the different types of HIV tests?

There are two main ways to diagnose HIV infection: detecting the virus directly
or detecting antibodies that are made against the virus.

The body makes antibodies to try to fight HIV, although the antibodies cannot
eradicate the virus. Antibody testing is often done in two parts. First a
sensitive screening test is performed on the blood. If the screening test is
positive, a second test is done to confirm that HIV antibodies are present. The
types of tests have varied over the years. At first, the screening test used an
enzyme-linked immunoassay (ELISA) with confirmatory testing by Western blot.
This strategy did not test for HIV-2, sometimes misclassified HIV-2 infection,
missed very early infections where antibody had not yet been produced, and
sometimes produced indeterminate results. Scientists developed newer tests to address
these issues. Other tests took hours or days to return results, requiring people
to return to the clinic. Rapid HIV tests (such as Clearview) were developed
that could provide results during the initial visit. Some tests can be done at
home without the need for a clinic visit. For a self-test or home-test, the
person buys a kit (for example, OraQuick), swabs the inside of their cheek, places the
swab in the supplied fluid, and reads the results in a test window. Positive
results indicate the need to visit a clinic for confirmatory testing.

New fourth-generation tests combine viral detection and antibody detection.
Viral detection is done by testing for a component of the virus known as p24
antigen. The fourth-generation tests also detect antibodies against both HIV-1 and
the less common HIV-2, as well as antibodies that are made in the early stage of
disease (IgM) and in chronic disease (IgG). Draft guidelines from the Centers
for Disease Control and Prevention (CDC) in 2012 recommend using FDA-approved
fourth-generation tests as the first step in diagnosis. If the fourth-generation
test is positive, additional antibody tests are done to differentiate HIV-1 from
HIV-2 and to measure viral load.

Viral load is measured by testing the amount of viral RNA in the blood. It
can be useful for patients who have confusing test results, such as a positive
fourth-generation test but negative or indeterminate individual tests for HIV-1 and
HIV-2. In these cases, if viral RNA is detected, the diagnosis of HIV is made. If
viral RNA tests are negative, it raises the possibility that the fourth-generation
test result was not correct. Viral load is also used to monitor the success of
treatment for infected patients. Viral load can also be useful in diagnosing
acute retroviral illness because HIV antibodies take time to reach detectable
levels.

The CDC recommends HIV testing at least once for all people between the ages
of 13 and 64. People at high risk, such as those who use illicit injectable
drugs or who have multiple sexual partners, should get tested more frequently.
All pregnant women should be tested because effective treatment can dramatically
reduce the risk of transmission to the unborn child. Victims of sexual assault
should be tested. People who are diagnosed with another sexually transmitted
disease, such as syphilis, chlamydia, or gonorrhea, should be tested for HIV as
well.